1. Field of the Invention
This invention is directed to methods for forming structures for microfluidic applications, and also to structures and devices formed by the methods.
2. Description of Related Art
Structures for microfluidic applications include passages in which fluids are contained and flowed. In order to provide controlled, uniform flow through the passages, it is important that the passages have well-defined characteristics.
One exemplary type of structure that includes fluid flow passages is the ink jet print head. Ink jet print heads typically include a base, an intermediate layer formed over the base, and a cover formed over the intermediate layer. The intermediate layer and cover form channels and nozzles for flowing and discharging the ink onto a recording medium to form images. Ink droplets are ejected from the nozzles by applying energy to the ink to form the droplets.
The intermediate layers in microfluidic devices include flow passages that can be defined by openings, or features, having various shapes and sizes, depending on their functions within the device.
Fluid passages have been formed in structural layers of microfluidic devices by different techniques. For example, photosensitized materials have been used for structural layers defining fluid flow passages in ink jet print heads. Photolithographic techniques have been used to form these fluid passages. However, such photolithographic techniques are not completely satisfactory for at least the following reasons. First, photolithographic techniques require the use of photosensitized materials. Thus, the group of suitable materials that can be patterned by conventional photolithographic techniques is limited. Second, in order to form accurate features by conventional photolithography techniques, very thin layers have been used. For thicker layers, the accuracy of photolithography is reduced. Third, it has been difficult to pattern features having different depths and/or widths in a photosensitized material by photolithography.
The features formed in a material can be characterized by their aspect ratio. The aspect ratio of a feature is determined by both its height and width. For a typical feature, however, there will also be a certain amount of taper of the side walls defining the feature. FIGS. 1 and 2 show two different opening configurations that have aspect ratios defined by respectively different relationships. FIG. 1 shows a layer 10 having a surface 12, and an opening 14 formed in the surface. The opening 14 has a height h and width w. The height h can be less than or equal to the thickness of the layer 10. The side walls 16 defining the opening 14 are perpendicular to the surface 12. For this opening configuration, the aspect ratio can be defined as the ratio of the height h to the width w of the opening; i.e.: h/w.
FIG. 2 shows a layer 20 formed on a substrate 22. The layer 20 has a thickness h and includes an upper surface 28, a lower surface 30, and an opening 32 extending between the upper surface 28 and the lower surface 30. The opening 32 is defined by side walls 34 which are tapered, such that the width of the opening 32 varies from a maximum width bxe2x80x2 at the upper surface 28 to a minimum width bxe2x80x3 at the lower surface 30. The layer 20 has a width axe2x80x2 at the upper surface 28 and a width axe2x80x3 at the lower surface 30. For the opening 32 having tapered side walls, the average aspect ratio of the opening 32 can be defined as: 2h/(bxe2x80x2+bxe2x80x3). Likewise, the average aspect ratio of the wall between the openings can be defined as: 2h/(axe2x80x2+axe2x80x3).
Structural layers in devices may require aspect ratios significantly greater than 1:1. In an ink jet print head, for example, features having aspect ratios significantly greater than 1:1, as well as features having significantly different aspect ratios, can be needed in different portions of the same device.
Conventional photolithography techniques have limited applicability for forming features that are tall, but narrow (i.e., have high aspect ratios) in thick photosensitized material layers. In addition, such techniques are unable to satisfactorily provide features having different heights in the same layer.
This invention provides methods for forming features in various different polymeric materials that can overcome the above-described disadvantages of known photolithographic techniques. Exemplary embodiments of the methods according to the invention can form fine features in non-photosensitized materials such embodiments can be used to form features in non-photosensitized materials that have not previously been achievable by known techniques. In addition, such methods can form fine features in non-photosensitized materials, for which photolithographic techniques are not suitable.
In addition, in exemplary embodiments of the methods according to the invention, fine features with high aspect ratios can be formed in non-photosensitized materials.
In addition, exemplary embodiments of the methods according to this invention can form features having different depths or widths in the same layer.
Furthermore, exemplary embodiments of the methods according to this invention can form features having different shapes and sizes in the same layer.
Thus, for example, in some embodiments, different portions of the same feature can have different depths, shapes and/or sizes. Accordingly, different portions of the same feature can provide different fluid flow characteristics. In addition, in some embodiments, different features can have different depths, widths, shapes and/or sizes in the same structural layer. Accordingly, different features of the same type can provide different fluid flow characteristics in the same structural layer. In addition, different types of features can be formed in the same structural layer to provide further versatility with respect to fluid flow.
Other exemplary embodiments of the methods according to the invention can form features in photosensitized materials by the combined use of laser ablation and photolithography. By combining these two different techniques, the patterning of features, or portions of features, that can be done by photolithography techniques can be performed by photolithography, while other features, or portions of the same feature, that previously have not been satisfactorily achieved in photosensitive materials by photolithography, can be formed by laser ablation. In embodiments, photolithography and laser ablation can be combined to form features in multi-layer structures including at least photosensitized material layer and at least one non-photosensitized material layer.
This invention also separately provides devices including such features.